Benthic bloom of cyanobacteria associated with fish mortality in Alexandria waters

A survey of the potentially harmful epiphytic microalgae along the coast of Alexandria was initiated since 2005 following an incident of mass mortality of the bottom feeding fish, Siganus rivulatus in the Eastern Harbour of Alexandria. S. rivulatus is known to feed on macroalgae specially Ulva sp.A total number of 132 samples were collected seasonally along Alexandria coast, from spring 2005 to summer 2010. Six cyanobacteria species are recorded; Oscillatoria acutissima, Oscillatoria nigroviridis, Oscillatoria limosa, Oscillatoria sp., Lyngbya sp. and Planktothrix c.f. agardhii. The causative organism responsible for the massive fish mortality is O. acutissima; with a standing crop reaching 1.07 × 106 filaments g−1 wet wt.The qualitative observations showed that Oscillatoria spp. seem to prefer green macroalgae as host to red and brown algae. The correlation between O. acutissima, O. nigroviridis and Ulva spp. showed to be significant (p = 0.002). While, no significant correlation was found between the other four species and the macroalgal species tested. The two species O. acutissima and O. nigroviridis as a result, were perennial and widespread during the present study.In the Egyptian coastal waters earlier dramatic events of harmful algae occurred on several occasions due to planktonic blooms of dinoflagellates. It is the first time that fish mortality is observed due to benthic cyanobacteria particularly O. acutissima. Our current work focuses on the possible impacts of cyanobacterial species on coastal food webs.